Continuing within the circle, tool-holders play an important role in hard milling. Because hard milling requires a large range of rpms-from low rpms for a roughing application to high rpm for HSM-only the HSK interface between the toolholder and spindle interface should be used.

This will provide a very rigid and balanced tooling setup over the ISO taper interface. .
The cutting tool can be held by several methods. These methods include collet chuck, hydraulic expansion, shrinkfit and power shrinking. The method selected should be determined by the requirements of the machining operation.

Collet chucks are by far the most flexible. In addition to offering maximum flexibility, they are easy to handle, provide excellent shock absorbing characteristics and offer an excellent range of clamping diameters. These are suitable for aggressive roughing and semifinishing of hardened materials.

Hydraulic expansion toolholders also provide ease of use as well as high clamping forces and minimal runout, which will provide extended cutting tool life. However, hydraulic tooling can be expensive and bulky to use. Similar to collect chucks, hydraulic tooling is an excellent choice for roughing and semifinishing operations.

For finishing those hardened cavities and cores with a high degree of accuracy and quality, power and heat-shrink toolholders provide excellent characteristics. Figure 5 outlines the pros and cons between all four systems. Today, all systems are commonly available from most tooling suppliers.

Cutting Tools
Although hard milling uses many aspects of HSM, the selection of appropriate cutting tools is most important in hard milling. Furthermore, cutting tools are a significant cost factor in both hard milling and HSM; making a good choice can help save money. One of the main contributing factors of hard milling failure is the cutting tool.

Many companies tend to skimp on selecting high-quality cutting tools, opting for less than adequate tools. To ensure that quality tools are selected, it is best to select an OEM who specializes in tools for hard milling or offers a well-defined product line for hard milling. The OEM should have technical staff on hand to assist in selecting the appropriate cutting tool for a particular hardened material and cutting strategy.

For roughing hardened materials, four-flute end mills or higher are recommended. This will provide small chip loads while having the ability to cut at higher feedrates. Additionally, torus end mills are recommended for roughing because the sharp edges of conventional end mills are not sufficiently resistant against the possibilities of vibration and thermal stress when cutting hardened materials.

The selection of cutting tools should be short with short flute lengths along with a helix angle of approximately thirty degrees. A thirty-degree helix has proven to be optimal for chip flow and dispersal of heat. The parent carbide substrate should also be considered.

Only fine or ultrafine grain sintered carbides should be used. Sintered hard carbide is a composite material based on powder metallurgy. A binder (usually cobalt) is used to bond carbide particles. Tungsten-, titanium-, tantal- or niobcarbide are the most used elements and provide the required hardness at high temperatures and wear resistance. With a reduction of the grain size of the carbide particles to about 0.5 to 0.6fm, the edge strength can be further increased, while the tendency to adhesion can be reduced.

For larger hardened cavities and cores, a selection of inserted cutting tools should be considered. Carbide inserts are less expensive than end mills and by rotating the insert, insert life can be extended. However, these tools are not typically designed for high spindle speeds and runout can be significant. There is also a significant safety risk if improper handling occurs.

Hard milling creates a great amount of stress on the tool from high heat and abrasive wear. To help overcome these stresses, coatings must be applied to the cutting tool. These coatings offer a protective layer on the tool, substantially increasing its life.

The most common coatings are titanium nitride (TiN), titanium carbon nitride (TiCN), titanium aluminum nitride (TiAlN) and titanium aluminum carbon nitride (TiAlCN); each coating has its benefits . Coating selection should be made based on individual properties and the OEM may dictate these when selecting a cutting tool. highlights the more important properties related to the coatings.

The titanium-based hard material layers such as TiCN and TiAlN are the most commonly used protection layers for HSM and hard milling cutting tools. The resistance to wear (hardness) is the most important property of TiCN, while TiAlN has a better heat and oxidation resistance property.

The OEM may also further enhance the coatings by offering unique blends, perhaps creating a leading edge over coating quality and tool life. Recently, other advances in coatings have entered the market such as proprietary coatings. Commonly called Rainbow coating, it is a proprietary multielement PVD (physical vapor deposit) coating offering a competitive edge over traditional nonpropriety coatings.

Flood coolants are traditionally used throughout the machining process to help disperse heat and remove chips from the work area and the cutting tool.

Hard milling often generates a tremendous amount of heat over conventional machining. This heat transferred into the chip and the use of flood coolant during hard milling causes the coolant to vaporize as it hits the hot chips. The use of coolants also can create thermal instability with the tool. Therefore, flood coolants are not commonly used in hard milling.

To help displace chips during the cutting process, compressed air is used. Additionally, a combination of oil/mist is often selected. The addition of oil helps reduce friction, therefore increasing tool life and surface finish. When using oil/mist, an oil/mist extraction unit should be integrated into the machine tool to help remove the oil from the air.